The invention relates to a sheet feeding motor controller for use in printers and the like. More particularly, the invention is directed to a method of and apparatus for maintaining constant torque of a motor used in a printer irrespective of any voltage variation in the printer's power supply.
Typically in serial printers, an astable power supply, such as a main secondary winding, is used as a power supply circuit. A voltage waveform, such as shown in FIG. 1, is observed as the output voltage (e.g., secondary voltage) of the power supply circuit. Upon start of a printing operation, the output voltage decreases from an initial no-load voltage V.sub.0 because the printing operation consumes a relatively large amount of power. At the end of the printing operation, the output voltage takes a value V.sub.s that is comparatively lower than the no-load voltage V.sub.0.
A sheet feeding operation is started almost simultaneously with the end of the printing operation. Since the power consumption during the sheet feeding operation is much smaller than the power supply capacity, the power supply can restore its output voltage level in accordance with an exponential curve characteristic of the power supply used. The restored voltage stabilizes at a voltage V.sub.e, which represents a difference between the no-load voltage V.sub.0 and the voltage drop due to driving the sheet feeding motor (e.g., 4 V due mainly to loss at a transformer). The voltage drop incurred during the printing process cyclically alternates with the voltage restoration (increase) during the sheet feeding process until the printing process is ultimately terminated.
The sheet feeding process requires accurate control over the sheet feeding amount, as well as the proper motor torque for proper sheet feeding. To meet such requirements, pulse motors are generally employed because they can control the sheet feeding amount easily and accurately. Pulse motors have the characteristic that a constant output torque can be maintained by controlling the rotational speed in proportion to a variation in the power supply voltage (if any).
A serial printer using an astable power supply therefore must control the rotational speed of a sheet feeding pulse motor. This control is typically performed by controlling the interval between the driving pulses used to control the pulse motor (i.e., controlling the pulse frequency). This control is made in accordance with the restoration curve of the power supply output voltage. However, the sheet feeding process involves a plurality of operations such as accelerating the pulse motor, driving the pulse motor at a constant torque, and decelerating the pulse motor. The speed of the sheet feeding pulse motor must therefore be controlled taking into account the restoration curve of the power supply output voltage with respect to each of these operations so that the best control can be implemented.
To achieve such a speed control, there is known a method of determining a proper speed (i.e., determining the interval between the pulses supplied to the pulse motor) by checking the output voltage of the power supply circuit every time the sheet feeding pulse motor is rotated for each operation.
This conventional method is excellent in achieving accurate torque control, but imposes the problem of overloading the arithmetic and logic circuits of the printer due to the complicated processing involved in calculating optimal speeds while checking the voltage during each operation.